WASHINGTON, May 4 (Reuters) - The U.S. House of
Representatives neared a cliffhanger vote on repealing
Obamacare on Thursday, with Republicans predicting victory on
overturning the healthcare law even though their seven-year
drive could founder in the U.S. Senate.

A House vote to repeal former President Barack Obama's
signature domestic achievement, which enabled 20 million more
Americans to get health insurance, would be President Donald
Trump's biggest legislative win since he took office in January.

However, it is by no means a sure thing either in the House,
where one Republican predicted a margin of only two votes in
favor, or in the Senate, where the Republican majority is narrow
and where lawmakers said the bill would face much greater
scrutiny and skepticism.

The House cleared a procedural hurdle paving the way to a
vote that was expected during the afternoon.

Despite controlling the White House and Congress,
Republicans have found that overturning Obamacare - which they
have long criticized as government overreach - is politically
fraught, in part because of voter fears that many people will
lose their health insurance as a result.

Passed in 2010, Obama's Affordable Care Act expanded
Medicaid, the government insurance program for the poor,
provided income-based tax credits to help the poor buy insurance
on individual insurance markets set up by the law, and required
everyone to buy insurance or pay a penalty. Republicans have
blamed it for driving up healthcare costs.

The Republican bill, called the American Health Care Act,
would repeal most Obamacare taxes, which paid for the law, roll
back the Medicaid expansion and slash the program’s funding,
repeal the penalty for not purchasing insurance and replace the
law’s tax credits with flat age-based credits.

"This is a step away from more government control of our
healthcare and our day-to-day lives, and a return to freedom for
all Americans," said Republican Representative Luke Messer.

An unusually emotional debate erupted on the House floor
Thursday morning as Democrats blasted the bill, saying it would
make insurance unaffordable for those who need it most and would
leave millions more uninsured.

"We don’t even know how much this bill will cost America,"
Democratic Representative Joseph Crowley said during a debate on
the House floor.

In a push to pass the bill before representatives leave on
Friday for a week in their home districts, the House will vote
before the bill can be assessed by the non-partisan
Congressional Budget Office (CBO), which estimates its cost and
effect on insurance rolls.

Republicans have said that the bill will be scored by the
CBO and other fixes will be made before the Senate votes.

In a sign of the scrutiny it will face there, Republican
Senator Bob Corker told MSNBC there was no way the healthcare
bill would receive a quick up-or-down vote in the Senate and
predicted senators would spend "at least a month" studying it.

Trump made overturning Obamacare a cornerstone of his 2016
campaign and has been frustrated as two efforts to push a
Republican bill through the House failed in the last two months,
a reflection of the difficulty of reconciling various Republican
Party factions.

PRE-EXISTING CONDITIONS

Representative Chris Collins predicted the bill would pass
by just two votes. If the bill passed by just one vote, Collins
suggested individual Republicans could come under attack in next
year's midterm election from voters labeling them as the reason
for its becoming law.

Wavering moderate Republicans had worried that the
legislation would undo a popular aspect of Obamacare and leave
too many people with pre-existing medical conditions unable to
afford health coverage.

But the skeptical Republican lawmakers got behind the bill
after meeting with Trump to float a compromise proposal, which
would add $8 billion over five years to help cover the cost for
people with pre-existing illnesses who could otherwise be priced
out of insurance markets.

Members of the Freedom Caucus, a faction of conservative
House lawmakers who played a key role in derailing the original
Republican version last month, said they could go along with the
compromise.

Healthcare consultancy and research firm Avalere Health said
in an analysis released on Thursday that the Republican bill
would only cover 5 percent of enrollees with pre-existing
conditions in the individual markets.

Nearly every major medical group, including the American
Medical Association, American Hospital Association and the AARP,
was strongly opposed to the Republican bill and said last-minute
amendments further eroded protection for the most vulnerable
groups, including the sick and elderly.

'COWARDLY CHOICE'?

House Democrats rejected the latest change to the Republican
legislation, saying it did not go far enough toward protecting
people with pre-existing conditions.

Nancy Pelosi, the top Democrat in the House, criticized
moderate Republicans who decided to support the bill with the
expectation it will be changed by the Senate.

"That is really a poor choice, a cowardly choice," she said.
"Why would they vote for it if they don’t think it is worthy of
support because the Senate might change it?"

Democrats have long thought their best chance of stopping
the repeal of Obamacare would be in the Senate. Republicans hold
a narrow 52-seat majority in the 100-seat chamber and so only
few Republican dissidents would be needed to stop the law from
moving forward.

Health insurers, such as Anthem Inc, UnitedHealth
Group Inc, Aetna Inc and Cigna Corp, have
faced months of uncertainty over healthcare's future. So have
hospital companies, such as HCA Holdings Inc and Tenet
Healthcare Corp.